Many of the molded products of which strength is required to be high, consist of fiber-reinforced plastics (FRP) in which a reinforcing fiber such as carbon fiber, aramid fiber and epoxy resin is combined.
As a forming process thereof, there are filament winding molding, drawing molding, transfer molding (RTM), autoclave molding using prepreg in which resin is impregnated into a reinforcing fiber beforehand, and a pressing molding.
Among them, RTM and a forming process similar to RTM is attracting attention because of excellent productivity thereof.
In accordance with RTM, textile-like continuous fiber is set in a mold at one time, and then a resin composition is poured into the mold to be cured, and hence the production efficiency is high. It should be noted that, as for RTM, it is necessary to decrease the viscosity of the resin composition to be poured, such that penetrating property of the resin into the reinforcing fiber increases. However, there is a problem in that epoxy resin is not suitable for RTM because it is difficult to decrease the viscosity of epoxy resin and it takes a long time for curing epoxy resin.
It is effective to heat the resin composition to be cured at a high temperature, in order to accelerate the curing of resin. However, when the molded product is large, it is likely to be difficult to heat for keeping it at a high temperature, and it has been difficult to mold it for a short time, because the temperature hardly increases in such a case. Moreover, when the molded product is large, it has been a large obstacle to install a heating apparatus, because a large-scale apparatus is required and costly.
On the other hand, an example using vinyl ester resin as a resin composition has been disclosed in Patent document 1 (one patent document reference). Since vinyl ester resin has a low viscosity, penetrating property thereof into reinforcing fiber is excellent and the chemical resistance of the resultant molded product will be high. In addition, it possible to cure vinyl ester resin at a lower temperature for a shorter time, compared to the case of using epoxy resin, because radical polymerization occurs using a peroxide catalyst therein. However, in general, when vinyl ester resin is used, the adherent property of the interface between the carbon fiber and the resin is inferior to the case of using epoxy resin, even if using carbon fiber which has been treated with a sizing agent for use in vinyl ester resin, and hence there is a problem in that the interlayer shearing strength deteriorates.
[Patent document 1]
Japanese Unexamined Patent Application, First Publication No. H06-088353